1. Field of the Invention
The invention relates generally to bearing assemblies for mud motors used in drilling of oil, gas, and water wells. More particularly, the invention relates to mud motor bearings for resisting on-bottom and off-bottom thrust loads.
2. Background of the Technology
In drilling a wellbore into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired wellbore. In conventional vertical wellbore drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the wellbore (or, in offshore drilling operations, on a seabed-supported drilling platform or a suitably adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry wellbore cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the wellbore wall (to stabilize and seal the wellbore wall). To optimize these functions, it is desirable for as much of the drilling fluid as possible to reach the drill bit.
Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to use “directional drilling” techniques to drill horizontal and other non-vertical wellbores, to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical wellbores. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a wellbore of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “mud motor”) incorporated into the drill string immediately above the drill bit. A typical mud motor includes several primary components, as follows (in order, starting from the top of the motor assembly):                a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);        a power section comprising a positive displacement motor of well-known type, with a helically-vaned rotor eccentrically rotatable within a stator section;        a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and        a bearing section comprising a cylindrical mandrel coaxially and rotatably disposed within a cylindrical housing; with an upper end coupled to the lower end of the drive shaft; and a lower end adapted for connection to a drill bit. Typically, the coupling to the drive shaft is accomplished by providing the upper end of the mandrel with a threaded “pin” connection that threads into the mating “box” connection of an adapter associated with the lower end of the drive shaft assembly.        
In drilling processes using a mud motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the mud motor to generate power to rotate the drill bit.
The bearing section must permit relative rotation between the mandrel and the housing, while also transferring axial thrust loads between the mandrel and the housing. Axial thrust loads arise in two drilling operational modes: “on-bottom” loading, and “off-bottom” loading. On-bottom loading corresponds to the operational mode during which the drill bit is boring into a subsurface formation under vertical load from the weight of the drill string, which in turn is in compression; in other words, the drill bit is on the bottom of the wellbore. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the wellbore and the drill string is in tension (i.e., when the bit is off the bottom of the wellbore and is hanging from the drill string, such as when the drill string is being “tripped” out of the wellbore, or when the wellbore is being reamed in the uphole direction). Tension loads across the bearing section housing and mandrel are also induced when circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly.
Accordingly, the bearing section of a mud motor must be capable of withstanding thrust loads in both axial directions, with the mandrel rotating inside the housing. A mud motor bearing section may be configured with one or more bearings that resist on-bottom thrust loads only, and with another one or more bearings that resist off-bottom thrust loads only. Alternatively, one or more bi-directional thrust bearings may be used to resist both on-bottom and off-bottom loads. A typical thrust bearing assembly comprises bearings (commonly but not necessarily roller bearings contained within a bearing cage) disposed within an annular bearing containment chamber. Suitable radial bearings (e.g., journal bearings or bushings) are used to maintain coaxial alignment between the mandrel and the bearing housing.
Thrust bearings contained in the bearing section of a mud motor may be either oil-lubricated or mud-lubricated. In an oil-sealed bearing assembly, the thrust bearings are disposed within an oil-filled reservoir to provide a clean operating environment. The oil reservoir is located within an annular region between the mandrel and the housing, with the reservoir being defined by the inner surface of the housing and the outer surface of the mandrel, and by sealing elements at the upper and lower ends of the reservoir.
Mud-lubricated bearing assemblies comprise bearings that are designed for operation in drilling fluid (“mud”). A small portion of the drilling fluid flowing to the drill bit is diverted to flow through the bearings to provide lubrication and cooling.
Oil-sealed bearing assemblies offer several advantages over mud-lubricated bearing assemblies. Because of the clean operating environment, oil-sealed components tend to have a much longer service life. Since conventional mud-lubricated bearing assemblies require a portion of the drilling fluid to be diverted through the bearings and to the wellbore annulus, the total flow of fluid through the drill bit is reduced, thereby reducing the effectiveness of the drilling fluid hydraulics through the bit. Oil-sealed assemblies do not require drilling fluid to be diverted and can be configured such that all the drilling fluid is directed through the bit, thus optimizing drilling fluid hydraulics through the bit. This can be particularly advantageous when running additional drilling tools between the mud motor and the drill bit, such as a rotary steerable system, where full flow of drilling fluid to the tool is required for optimum operation.
However, mud-lubricated bearings have their own advantages. In particular, mud-lubricated bearings with planar bearing contact surfaces can provide static thrust load capacities considerably greater than is achievable with conventional rolling-element bearings. In addition, mud-lubricated bearings can operate reliably in harsh environments, without need for a sealed bearing chamber.
As previously noted, separate thrust bearings may be used for on-bottom and off-bottom thrust loads, or bi-directional thrust bearings may be used to resist both on-bottom and off-bottom thrust loads. In either case, the mandrel must incorporate a load-transferring shoulder situated above the off-bottom bearing, for transferring off-bottom loads from the mandrel to the housing. This is commonly accomplished in prior art bearing assemblies through the use of a ring machined with an array of high-tolerance annular grooves and ribs sized to mate with corresponding high-tolerance annular ribs and grooves on the mandrel. The ring is necessarily provided in the form of a split ring to allow assembly onto the mandrel. When assembled on the mandrel, the split ring provides the necessary shoulder for off-bottom loads, which are transferred from the off-bottom thrust bearing (or, alternatively, a bi-directional thrust bearing) to the mandrel through the mating annular grooves and ribs of the mandrel and split ring. The spacing of the grooves and ribs in the mandrel and the split ring must be very precise so that axial load is shared equally between each adjacent set of mating groove/rib faces.
A rolling-element bearing (i.e., a bearing incorporating any type of rolling element, such as balls, cylindrical rollers, tapered rollers, and spherical rollers) will have static and dynamic load ratings that define allowable load limits during operation. An off-bottom thrust bearing can experience high static loads if the drill bit becomes stuck in the wellbore and the drill string needs to be put in tension in an attempt to pull the bit free. If the static load limit of the off-bottom bearing is exceeded, the motor will not be operable once the bit is pulled free, and the motor will need to be removed from the wellbore and replaced before drilling can continue.
For at least the reasons discussed above, there remains a need in the art for an oil-sealed mud motor bearing section in which the mandrel is provided with a load-transferring shoulder for reacting off-bottom thrust loads, but without the need for high-tolerance machining of the mandrel and associated shoulder components. Further, there remains a need in the art for a mud motor bearing section incorporating an off-bottom thrust bearing having a static load limit much greater than provided by rolling-element bearings. Still further, there remains a need in the art for a mud motor bearing section incorporating a mud-lubricated off-bottom bearing assembly in which the mud flow through the off-bottom bearing assembly is returned to the main mud flow through the bearing section, rather than exiting into the wellbore annulus and thereby reducing the total mud flow reaching the drill bit. Embodiments disclosed herein are directed to such needs.